This invention relates generally to the selective deposition of a material onto a substrate, and more particularly to a method and system for the deposition of a continuous stream of laser-melted powdered material to form complex, net-shape objects.
Since their development in the early 1960's, lasers have evolved to a practical tool for industrial materials processing. In the last decade, laser cladding of materials has proven to be a technology which satisfies industrial requirements for a variety of applications. In particular, laser cladding of similar or dissimilar materials for improved wear properties, corrosion resistance, thermal barrier coatings, etc. has been developed. R. Subramanian et al. in J. Material Science 26, 951 (1991), and C. Xin et al. in Key Engineering Materials 46 & 47, 381 (1990) describe laser cladding in which a surface is exposed to a laser beam, and powder is fed from a single off-axis position onto the surface where the powder is melted and bonded to the surface. Typical applications are for hardfacing and improved corrosion resistance of the substrate. More recently, a process similar to laser cladding for generating solid metal objects has been described wherein laser powder or wire deposition to rebuild and/or produce solid geometries has been used. U.S. Pat. No. 4,323,756 teaches the building up of solid articles by impinging an energy beam onto a surface sufficient to melt it, and introducing powder or wire feedstock into the molten zone from a single off-axis position.
In the development of the laser deposition system of the present invention, it was found necessary to introduce the powdered material into the laser beam at the precisely correct position. With previous single-port, off-axis nozzle laser-powder deposition techniques, low-frequency periodic fluctuations in the powder flow rate lead to non-uniform deposition regions. When the low-frequency fluctuations occurred, a comb-like periodic structure was generated in the deposited material. These fluctuations resulted in variations in deposition-surface height and introduced instabilities in laser and powder interactions at the workpiece, producing inconsistent results. Since the laser beam can be significantly attenuated or even defocused by injection of the powdered material into the beam at a point other than minimum beam diameter, it is necessary to introduce the powdered material with some precision. In particular, we have discovered that introducing a converging stream of powdered material at an angle as nearly normal to the work surface as possible, and at or near the minimum laser beam diameter can dramatically improve the quality of the resulting objects. Additionally, we have found that it is important to inject the powdered material symmetrically into the laser beam independent of the direction of relative motion of the workpiece with respect to the laser beam so as to produce objects with uniform geometries. The laser deposition system of this invention satisfies all the foregoing needs providing a method for direct manufacture of complex metallic, ceramic, glass and plastic as well as composite net-shape objects.
The present invention satisfies the long felt need for a successful laser deposition process in which a stream of powder transported in a gas intersects a focused laser beam; is melted thereby, either in flight or upon injection into a molten puddle, and is deposited onto a substrate. Scanning the laser beam over a deposition stage, or moving the deposition stage relative to the laser beam, while maintaining a sharp focus on or near the surface being deposited thereon allows the generation of solid objects of varying geometry in a layer-wise manner.